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Thelyphonida, in dorsal view, with pedipalps highlighted in green

Androctonus scorpion showing the comparatively slender pedipalps (pincers) indicative of a highly venomous species

Pedipalps, the second pair of appendages of the cephalothorax in Arachnida, is homologous with mandibles in Crustacea and corresponding to the mandibles of insects. The pedipalps are appendages of six segments: the coxae, a single trochanter, the femur, a short patella, the tibia, and the tarsus. The coxae, most frequently have extensions, called maxillae or gnahobases, which function as mouth parts with or without contribution from the coxae of the anterior legs. The limbs themselves may be simple tactile organs outwardly resembling the legs, as in spider, or chelate weapons of great size, as in the scorpion. Comparative studies of pedipalpal morphology may suggest that leg-like pedipalps are primitive in Arachnida. At present, the only reasonable alternative to this view is to assume that xiphosurans reflect the morphology of the primitive arachnid pedipalp and to conclude that this appendage is primitively chelate. Pedipalp chelae are found in several arachnid groups, Ricinulei, Thelyphonida, Scorpiones and Pseudoscorpiones, but the chelae in most of these taxa do not appear to be homologous with those found in Xiphosura. The pedipalps are distinctly raptorial in Amblypygi, Thelyphonida, Schizomida and Opiliones.

Araneus diadematus male showing two pedipalps

male and female pedipalps in Zygiella x-notata

Pedipalps of spiders have the same segmentation as the legs, but the tarsus is undivided, and the pretarsus has no lateral claws. A male spider bears enlarged ends of his pedipalps, the terminal segments of which are elaborated into organs for the transfer of sperm to the sperm receptacles of the female. A microscopic analysis of the pedipalps is often the only way to exactly determine the species of a male spider.

• Savory, T. 1977. Arachnida. 2nd edition. U.S. Edition published by Academic Press INC. LTD.340 Pp.

• Snodgrass, R.E. 1971. A Textbook Arthropod Anatomy. Published by Hafner Publishing Company, INC. 363 Pp.

• Torre-Bueno, J.R. 1989. The Torre-Bueno Glossary of Entomology/ compiled by Stephen W. Nichols; including Supplement A by George S. Tulloch. Published by The New York Entomological Society in cooperation with the American Museum of Natural History. 840 Pp.

• Discovery Books; 2000; Insects and Spiders; St. Remy Media Inc.; New York; 35

• Close-up view of sperm-laden pedipalps in a mature tarantula Several ad popups warning!
• Several close-up photos of a tarantula creating a sperm web - Brachypelma Vagans (Mexican Red-rump)